As a semiconductor device becomes more integrated, a multi-layered process is typically used. Photolithography processes are utilized in the multi-layered process, and ever smaller critical dimension margins are sought. To help minimize a line width formed on a material layer, the material layer on a chip is globally planarized. Currently, methods for planarizing a semiconductor device include boro-phospho-silicate glass (BPSG) reflow, aluminum (Al) flow, spin on glass (SOG) etch back, and chemical mechanical polishing (CMP).
CMP uses chemical components in a slurry solution and physical components of a polishing pad to chemically and mechanically polish the surface of a chip for planarization. This enables CMP to achieve global planarization and low-temperature planarization for a broad area, where a reflow process or an etch-back process is not able to be performed. Due to these advantages CMP is widely used as a planarization technique for next-generation semiconductor devices.
In a related art CMP apparatus, a nozzle supplies slurry while a pad rotates at a predetermined speed. A carrier applies a predetermined pressure on a wafer attached to the pad, and rotates at a predetermined speed.
A deposited layer on a wafer can be polished by this CMP process. The rotating pad, rotating carrier, and pressure on the wafer serve as physical components, while the slurry chemically interacts with the layer deposited on the wafer.
Performing the CMP polishing process often leads to the pad becoming smoother and losing surface roughness. If the surface roughness of the pad is not restored to its former condition, the polishing speed and uniformity during the subsequent processes will be degraded.
In order to provide additional surface roughness and to supply new slurry to the pad between polishing processes, the pad is typically pressed in a predetermined conditioning pressure by using a rotating circular disk.
FIG. 1 is a view of a related art CMP apparatus.
Referring to FIG. 1, a wafer 100 is polished by a pad 110 and slurry 120, and a polishing table 130 attached to the pad 110 performs a simple rotating movement. A head 140 also performs a rotating movement and applies a predetermined pressure on the wafer 100.
The wafer 100 uses a pad conditioner to condition the surface of the pad 110 such that the damage of the pad 110 after polishing can be recovered. Then, the next wafer is processed.
FIG. 2 is a top view of a head and a pad in a CMP apparatus. FIG. 3 is a graph of rotating speed with respect to wafer radius. FIG. 4 is a graph of polishing rate with respect to wafer radius.
As illustrated in FIG. 2, when the pad 110 and the head 140 rotate in the same direction, the rotating speed increases as points of the pad 110 are located closer to the outer circumference of the pad 110. Therefore, the polishing rate of a wafer disposed below the head 140 also increases as the radius of the pad 110 is closer to the outer circumference.
More specifically, as illustrated in FIG. 4, the polishing rate increases from the center of the wafer to the outer circumference. Furthermore, the rate at which the polishing rate increases also goes up from the center to the outer circumference of the wafer 100. This occurs because the head applies different pressure on the wafer, which is caused by different rotating speeds (distance per time unit) at each point.
The rotating speed increases from the center toward the outer circumference of the wafer such that the edge portion is more polished than the center of the wafer.
When the pad 110 and the head 140 rotate, the wafer is not uniformly polished. This leads to irregularities in the semiconductor device being polished and deterioration of its characteristics. Thus, there exists a need in the art for an improved CMP technique for planarizing a semiconductor device.